Ground fault circuit interrupting (GFCI) devices have been sanctioned by the National Electric Code for use in residential circuits to protect against the hazards of electrical shock due to ground faults. Such GFCI devices, as presently constructed, utilize a differential current transformer to sense a current imbalance in the line and neutral conductors of a distribution circuit as occasioned by current flowing through a line-ground fault and returning to the source through an unintended ground circuit path other than the neutral conductor. To provide effective electrical shock protection, the differential current transformer must develop a signal of sufficient magnitude to enable a signal processor to initiate circuit interruption when the current imbalance in the line and neutral conductors is 5 milliamps or more. For ease of manufacture and to accommodate a compact design, the line and neutral conductors, which constitute the primary windings of the differential current transformer, preferably each make a single pass through the aperture of the toroidal transformer core. Thus, to satisfy a 5 milliamp trip level, the GFCI signal processor must be designed to respond to a transformer primary excitation of 0.005 ampere-turns. So that the design constraints on the signal processor are not so rigid as to be prohibitively expensive, the differential current transformer must have a high permeability core and a secondary winding of many turns, typically in excess of 1,000 turns of very fine wire, in order to develop a fault signal voltage across a burden resistor of practical magnitudes. Signal levels are nevertheless quite low, one to ten millivolts, requiring high amplification. With such high amplification, the processor design must insure amplifier stability and adequate noise immunity to prevent nuisance tripping of the GFCI device.
In addition to sensing the existence of ground fault current flowing through a high impedance line-ground fault, approved GFCI devices must also respond to the existence of a low impedance ground fault on the neutral conductor adjacent the load. Since the neutral conductor is also grounded at the source in conventional wiring installations, such double grounding of the neutral conductor creates the situation where at least a portion of any ground fault current flowing through a line-ground fault returns to the source via the neutral-ground fault and the neutral conductor. As a consequence, the current differential or imbalance showing up in the differential current transformer would not truly be indicative of the magnitude of the ground leakage current. Consequently, the GFCI device would trip only in response to considerably higher ground fault current levels. It is for this reason that Underwriters Laboratories requires that GFCI devices also have the capability of interrupting the circuit in the event of desensitizing, low impedance ground fault on the neutral conductor.
The conventional approach toward coping with such a desensitizing neutral-ground faults to utilize a second, so-called neutral transformer having a secondary winding connected in series with at least the neutral side of the distribution circuit. If the neutral conductor experiences a low impedance ground fault adjacent the load, it becomes a closed loop secondary winding, and driving of the neutral transformer primary winding will produce a current flow in this secondary loop. If the neutral ground fault impedance is sufficiently low as to have a significant desensitizing effect on the response of the GFCI device to ground fault current, the current induced in the secondary loop is of sufficient magnitude to create the requisite current imbalance in the differential transformer for initiating a trip function.
The addition of this second transformer adds significantly to the cost of current GFCI designs, and it also takes up valuable space which is at a premium when faced with the task of packaging a GFCI module in circuit breakers and convenient outlet receptacles capable of being installed in existing enclosures.
It is accordingly an object of the present invention to provide an improved ground fault circuit interrupting device capable of responding both to high impedance ground faults on the line conductor and low impedance ground faults on the neutral conductor of a conventional electrical power distribution circuit.
An additional object is to provide a ground fault circuit interrupting device of the above character wherein both ground fault sensing functions are accommodated utilizing a single current transformer.
A further object is to provide a ground fault circuit interrupting device of the above character which utilizes an improved signal processor design approach to the handling of ground fault signals appearing in the secondary of the single current transformer.
Still another object of the present invention is to provide a ground fault circuit interrupting device of the above character, wherein the single transformer may be of an inexpensive design in the sense that its toroidal core may have a lower permeability and its secondary winding fewer turns as compared with current transformer designs.
Yet another object of the present invention is to provide a ground fault circuit interrupting device of the above character, which is reliable in operation, not prone to nuisance tripping, and inexpensive to manufacture in quantity.
Other objects of the invention will in part be obvious and in part appear hereinafter.